This invention relates to the production of stretch plastic film.
Stretch plastic film, commonly known as stretch wrap, has gained substantial acceptance for such uses as warehouse packaging applications where plastic film is stretched around pallets, containers or irregular loads, with a built-in elastic recovery properties of the film constraining the surrounding item(s). A number of plastic materials, such as PVC, LLDPE, LDPE, and EVA/LDPE, are used to produce stretch film for commercial use.
The properties of the stretch film obtained are dependent upon a large number of variables, such as the extrusion process, film thickness, monolayer or multilayer film, cooling rate, blow up ratio and stretch ratio. Currently, extensive research work is being carried out in industrial laboratories to improve the properties of stretch film, such as balance of peel-lap cling, tensile strength, tear resistance, transparency, etc.
Most stretch films are produced by a cast film process. However, in such a process, orientation is effected only in the machine direction. Tensile strength can be improved if the film is produced by a blown film process. Also, due to the inherent nature of the cast film process, edge trimming is essential and this can lead to kinks on the edges, thereby making the film vulnerable to tear. Tear properties can also be improved using a blown film process.
To improve the above mentioned properties of stretch film and increase production yield, a blown film process for stretch film was developed and is the subject of parent U.S. Pat. No. 6,162,318. In the invention disclosed and claimed in the parent application, a bubble formed from blown film is blocked intentionally when collapsed by using a relatively low tower height, i.e. a relatively short distance from extrusion die to collapsing frame, blocking meaning that opposed sides of the collapsed bubble become bonded together. The invention described and claimed in the parent application has various advantages but also has certain disadvantages, for example, low bubble stability at very thin gauges which results in reduction in output and also puts a lower limit on final blocked thickness of the film.
It is therefore an object of the invention to provide an improved method for the production of relatively thin stretch plastic film.
According to the invention, stretch plastic film is formed by extruding suitable plastic material from an annular die as a tubular film, forming the extruded film into a bubble with air entrapped therein by collapsing the tubular film at a predetermined distance from the annular die, causing the temperature of the tubular film when collapsed to be sufficiently high to cause opposite sides of the collapsed film to become bonded together to form a blocked two layer film, and stretching the blocked two layer film to reduce its thickness.
In addition to the advantages of a stretch film produced in accordance with the invention disclosed and claimed in parent U.S. Pat. No. 6,162,318, the present invention enables thin stretch film to be produced by means of a blown film process in which the bubble is relatively thick, with consequent improved bubble stability and hence higher output.
The stretching of the blocked two layer film may be effected by passing the blocked two layer film between a pair of driven stretch/nip rolls rotated at a faster peripheral speed than the speed of the collapsed film at said predetermined distance from the annular die. The blocked two layer film may be passed successively through a plurality of pairs of driven stretch/nip rolls, each subsequent pair of stretch/nip rolls being driven at a faster peripheral speed than the previous pair of stretch/nip rolls.
The blocked two layer film may be stretched by an amount in the range of from about 100 to about 400%, for example from a thickness in the range of from about 20 to about 30 microns to a thickness in the range of from about 10 to about 25 microns.